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Security LAKE Working Group Internet-Draft This document provides considerations for guiding the implementation of the authenticated key exchange protocol Ephemeral Diffie-Hellman Over COSE (EDHOC). Discussion Venues Discussion of this document takes place on the Lightweight Authenticated Key Exchange Working Group mailing list (lake@ietf.org), which is archived at . Source for this draft and an issue tracker can be found at .

Introduction Ephemeral Diffie-Hellman Over COSE (EDHOC) is a lightweight authenticated key exchange protocol, especially intended for use in constrained scenarios. During the development of EDHOC, a number of side topics were raised and discussed, as emerging from reviews of the protocol latest design and from implementation activities. These topics were identified as strongly pertaining to the implementation of EDHOC rather than to the protocol in itself. Hence, they are not discussed in , which rightly focuses on specifying the actual protocol. At the same time, implementers of an application using the EDHOC protocol or of an "EDHOC library" enabling its use cannot simply ignore such topics and will have to take them into account throughout their implementation work. In order to prevent multiple, independent re-discoveries and assessments of those topics, as well as to facilitate and guide implementation activities, this document collects such topics and discusses them through considerations about the implementation of EDHOC. At a high-level, the topics in question are summarized below:

• Handling of completed EDHOC sessions when they become invalid and of application keys derived from an EDHOC session when those become invalid. This topic is discussed in .
• Retention of completed EDHOC sessions that are still valid, also in the case that an EDHOC error message is received after their completion. This topic is discussed in .
• Enforcement of different trust policies, with respect to learning new authentication credentials during an execution of EDHOC. This topic is discussed in .
• Branched-off side processing of incoming EDHOC messages, with particular reference to: i) fetching and validation of authentication credentials; and ii) processing of External Authorization Data (EAD) items, which in turn might play a role in the fetching and validation of authentication credentials. This topic is discussed in .
• Effectively using EDHOC over the Constrained Application Protocol (CoAP) in combination with Block-wise transfers for CoAP , possibly together with the optimized EDHOC execution workflow defined in . This topic is discussed in .

The scope of the present implementation considerations only includes the use of EDHOC with the authentication methods specified in and based on public key authentication. A future document can extend the present document and include implementation considerations that consider the use of EDHOC with other authentication methods, such as the one defined in and based on symmetric pre-shared keys.

Terminology The reader is expected to be familiar with terms and concepts related to the EDHOC protocol , (CoAP) , and Block-wise transfers for CoAP .

Handling of Invalid EDHOC Sessions and Application Keys This section considers the most common situation where, given a certain peer, only the application at that peer has visibility and control of both:

• The EDHOC sessions at that peer; and
• The application keys for that application at that peer, including the knowledge of whether they have been derived from an EDHOC session, i.e., by means of the EDHOC_Exporter interface after the successful completion of an execution of EDHOC (see ).

Building on the above, the following expands on three relevant cases concerning the handling of EDHOC sessions and application keys, in the event that any of those becomes invalid. To provide more concrete guidance, the following considers the case where "application keys" stands for the keying material and parameters that compose a Security Context for the security protocol Object Security for Constrained RESTful Environments (OSCORE) , i.e., when specifically those application keys are derived from an EDHOC session (see ). Nevertheless, the same considerations are applicable if EDHOC is used to derive other application keys, e.g., when used to key different security protocols than OSCORE or to provide the application with secure values that are bound to an EDHOC session.

EDHOC Sessions Become Invalid The application at a peer P may have learned that a completed EDHOC session S has to be invalidated. When S is marked as invalid, the application at P purges S and deletes each set of application keys (e.g., the OSCORE Security Context) that was generated from S. Then, the application runs a new execution of the EDHOC protocol with the other peer. Upon successfully completing the EDHOC execution, the two peers derive and install a new set of application keys from this latest EDHOC session. The flowchart in shows the handling of an EDHOC session that has become invalid.

Handling of an EDHOC Session that Has Become Invalid. and the application keys ---> EDHOC ---> install new session derived from it application keys ]]>

An EDHOC session may have become invalid, for example, because an authentication credential CRED_X has expired, or because the peer P has learned from a trusted source that CRED_X has been revoked. This effectively invalidates CRED_X, and therefore also invalidates any EDHOC session where CRED_X was used as authentication credential associated with either peer in the session (i.e., P itself or the other peer). In such a case, the application at P has to additionally delete CRED_X and any stored, corresponding credential identifier.

Application Keys Become Invalid The application at a peer P may have learned that a set of application keys is not safe to use anymore. When such a set is specifically an OSCORE Security Context, the application may have learned that from the OSCORE library used or from an OSCORE layer that takes part in the communication stack. A current set SET of application keys shared with another peer can become unsafe to use, for example, due to the following reasons:

• SET has reached a pre-determined expiration time;
• SET has been established to use for an amount of time that is now elapsed, according to enforced application policies; or
• Some elements of SET have been used enough times to approach cryptographic limits that should not be passed, e.g., according to the properties of the security algorithms specifically used. With particular reference to an OSCORE Security Context, such limits are discussed in .

When this happens, the application at the peer P proceeds as follows:

1. If the following conditions both hold, then the application moves to Step 2. Otherwise, it moves to Step 3.
   • Let us define S as the EDHOC session from which the peer P has derived SET or the eldest SET's ancestor set of application keys. Then, since the completion of S with the other peer, the application at P has received from the other peer and successfully verified at least one message protected with any set of application keys derived from S. That is, P has persisted S (see ).
   • The peer P supports a key update protocol, as an alternative to performing a new execution of EDHOC with the other peer. When SET is an OSCORE Security Context, the key update protocol supported by the peer P can be KUDOS .
2. The application at P runs the key update protocol mentioned at Step 1 with the other peer, in order to update SET. When SET is an OSCORE Security Context, the application at P can run the key update protocol KUDOS with the other peer. If the key update protocol terminates successfully, the updated application keys are installed and no further actions are taken. Otherwise, the application at P moves to Step 3.
3. The application at the peer P performs the following actions:
   • It deletes SET.
   • It deletes the EDHOC session from which SET was generated, or from which the eldest SET's ancestor set of application keys was generated before any key update occurred (e.g., by means of the EDHOC_KeyUpdate interface defined in or other key update methods).
   • It runs a new execution of the EDHOC protocol with the other peer. Upon successfully completing the EDHOC execution, the two peers derive and install a new set of application keys from this latest EDHOC session.

The flowchart in shows the handling of a set of application keys that has become invalid. In particular, it assumes such a set to be an OSCORE Security Context and the key update protocol to be KUDOS.

Handling of a Set of Application Keys that Has Become Invalid. Delete the application ----> Rerun application keys keys and the EDHOC session EDHOC persisted? ^ ^ | | | | | | YES | | v v | | | | Derive and install Is KUDOS NO | | new application keys supported? ------------------+ | | | | | YES | v | | Run KUDOS | | | | | | v | | Has KUDOS NO | succeeded? ---------------------------+ | | YES v Install the updated application keys ]]>

Application Keys or Bound Access Rights Become Invalid The following considers two peers that use the ACE framework for authentication and authorization in constrained environments and specifically the EDHOC and OSCORE profile of ACE defined in . When doing so, one of the two peers acts as ACE resource server (RS) hosting protected resources. The other peer acts as ACE client and requests an access token from an ACE authorization server (AS) that is in a trust relationship with the RS. The access token specifies access rights for accessing protected resources at the RS as well as the public authentication credential of the client, namely AUTH_CRED_C. After that, C uploads the access token to the RS, by means of an EAD item included in an EDHOC message during the EDHOC execution (see below). Alternatively, the AS can upload the access token to the RS on behalf of the client, as per the alternative Short Distribution Chain (SDC) workflow defined in . Consistent with the EDHOC and OSCORE profile of ACE, the two peers run EDHOC in order to specifically derive an OSCORE Security Context as their shared set of application keys (see ). At the RS, the access token is bound to the successfully completed EDHOC session and to the established OSCORE Security Context. After that, the peer acting as ACE client can access the protected resources hosted at the other peer acting as RS, according to the access rights specified in the access token. The communications between the two peers are protected by means of the established OSCORE Security Context. Later on, the application at one of the two peers P may have learned that the established OSCORE Security Context CTX is not safe to use anymore, e.g., from the OSCORE library used or from an OSCORE layer that takes part in the communication stack. The reasons that make CTX not safe to use anymore are the same ones discussed in when considering a set of application keys in general, plus the event that the access token bound to CTX becomes invalid (e.g., it has expired or it has been revoked). When this happens, the application at the peer P proceeds as follows.

1. If the following conditions both hold, then the application moves to Step 2. Otherwise, it moves to Step 3:
   • The access token is still believed to be valid. That is, it has not expired yet and the peer P is not aware that it has been revoked.
   • Let us define S as the EDHOC session from which the peer P has derived CTX or the eldest CTX's ancestor OSCORE Security Context. Then, since the completion of S with the other peer, the application at P has received from the other peer and successfully verified at least one message protected with any set of application keys derived from S. That is, P has persisted S (see ).
2. If the peer P supports the key update protocol KUDOS , then P runs KUDOS with the other peer, in order to update CTX. If the execution of KUDOS terminates successfully, the updated OSCORE Security Context is installed and no further actions are taken. If the execution of KUDOS does not terminate successfully or if the peer P does not support KUDOS altogether, then the application at P moves to Step 3.
3. The application at the peer P performs the following actions.
   • If the access token is not believed to be valid anymore, the peer P deletes all the EDHOC sessions associated with the access token as well as the OSCORE Security Context derived from each of those sessions. Note that, when specifically considering the EDHOC and OSCORE profile of ACE, an access token is associated with at most one EDHOC session (see ). If the peer P acted as ACE client, then P obtains from the ACE AS a new access token, which is uploaded to the other peer acting as ACE RS. Finally, the application at P moves to Step 4.
   • If the access token is valid while the OSCORE Security Context CTX is not, then the peer P deletes CTX. After that, the peer P deletes the EDHOC session from which CTX was generated, or from which the eldest CTX's ancestor OSCORE Security Context was generated before any key update occurred (e.g., by means of KUDOS or other key update methods). Finally, the application at P moves to Step 4.
4. The peer P runs a new execution of the EDHOC protocol with the other peer. Upon successfully completing the EDHOC execution, the two peers derive and install a new OSCORE Security Context from this latest EDHOC session. At the RS, the access token is bound to this latest EDHOC session and the newly established OSCORE Security Context.

The flowchart in shows the handling of an access token or of a set of application keys that have become invalid, when using the EDHOC and OSCORE profile of the ACE framework.

Handling of an Access Token or of a Set of Application Keys that Have Become Invalid. Delete the associated --> Obtain and --> Rerun ---+ access token EDHOC sessions and upload a EDHOC | still believed the application keys new access | to be valid? derived from those token ^ | | | | | | | | | | | YES | | v | | | | The application keys | | are not valid anymore | | | | | | | | | | v | | | | Are the NO | | application keys -----> Delete the application keys and ------+ | persisted? the associated EDHOC session | | | ^ ^ | | YES | | | v | | | | | | Is KUDOS NO | | | supported? ---------------------+ | v | | | Derive and install | YES | new application keys v | | Run KUDOS | | | | | | v | | Has KUDOS NO | succeeded? ------------------------------+ | | YES v Install the updated application keys ]]>

Retention of Completed EDHOC Sessions After successfully completing an EDHOC session S and possibly using the EDHOC_Exporter interface to derive keying material from S, an EDHOC peer is expected to store and retain the latest state of S over time. Requirements to fulfill for persistently storing PRK_out or derived application keys are defined in Sections and of . Retaining the state of S ensures that it is possible to:

• Update S by updating its associated PRK_out in a more efficient way than re-running EDHOC, e.g., by using the EDHOC_KeyUpdate function defined in .
• Use the EDHOC_Exporter interface for late derivations of keying material from S that cannot be performed shortly after the session completion or according to a predictable schedule.

Absent application policies defining more restrictive lifetimes, the peer is expected to retain the latest state of S in its local storage until:

• S has to be deleted due to reasons discussed in ; or
• S has to be deleted due to memory limitations, in which case the peer ought to delete the oldest completed EDHOC session first.

Handling of Incoming EDHOC Error Messages defines that an EDHOC session is completed after having successfully processed the last message, i.e., message_3 or message_4, depending on the application profile used (see ). It follows that:

• The Responder completes a session after successfully processing the incoming EDHOC message_3, if EDHOC message_4 is not used, or after sending EDHOC message_4 otherwise.
• The Initiator completes a session after successfully processing the incoming EDHOC message_4, if EDHOC message_4 is used, or after sending EDHOC message_3 otherwise.

Furthermore, defines EDHOC error messages and the processing associated with the initial set of error codes. According to , after an EDHOC session is completed, no EDHOC error messages are sent, and the EDHOC session output may be maintained even if EDHOC error messages are received. That is, an implementation has a lot of latitude about handling incoming EDHOC error messages that pertain to a completed EDHOC session. In general, a safe approach simply consists in aborting the completed EDHOC session, thereby deleting the corresponding output such as derived application keys. If the reception of EDHOC error messages at a given peer is still plausible, this is actually an appropriate course of action for that peer. An example is the case where the Responder sends EDHOC message_4, and it receives an EDHOC error message as a follow-up from the Initiator that rejected EDHOC message_4. However, there are indeed cases where it is not plausible anymore to receive EDHOC error messages pertaining to a completed EDHOC session. Such EDHOC error messages can be safely ignored as irrelevant and possibly resulting from an attack, thereby preserving the EDHOC session output such as derived application keys. An example is the case where the Responder successfully processes the incoming EDHOC message_3 and, at the same time, EDHOC message_4 is not used. To provide more concrete guidance, the rest of this section considers the case where the following applies:

• "application keys" stands for the keying material and parameters that compose an OSCORE Security Context , i.e., when specifically those application keys are derived from an EDHOC session (see ).
• EDHOC messages are transferred over CoAP using the forward message flow (see ), i.e., the EDHOC Responder acts as a CoAP server and the EDHOC Initiator acts as a CoAP client.

Building on the above, the following holds for the EDHOC Responder.

• If EDHOC message_4 is not used, the Responder completes the EDHOC session after successfully processing the incoming EDHOC message_3. In this case, no further EDHOC messages are supposed to be exchanged in the session. Consequently, the Responder can safely set the EDHOC session to ignore any incoming EDHOC error message pertaining to the session from then on, thereby preserving the OSCORE Security Context derived from the session.
• If EDHOC message_4 is used, the Responder completes the EDHOC session after sending EDHOC message_4. After that, it remains generally appropriate for the Responder to abort the EDHOC session in the event that the Responder receives an EDHOC error message pertaining to the session. In particular, the EDHOC error message might have been legitimately sent by the Initiator that failed to process EDHOC message_4. However, the Responder can safely set the EDHOC session to ignore any incoming EDHOC error message pertaining to the session from then on, after successfully processing an incoming OSCORE-protected message received from the Initiator. Similarly to the case previously discussed, this preserves the OSCORE Security Context derived from the session, in the event that the Responder receives EDHOC error messages.

Also building on the above, the following holds for the EDHOC Initiator.

• If EDHOC message_4 is used, the Initiator completes a session after successfully processing the incoming EDHOC message_4. In this case, no further EDHOC messages are supposed to be exchanged in the session. Consequently, the Initiator can safely set the EDHOC session to ignore any incoming EDHOC error message pertaining to the session from then on, thereby preserving the OSCORE Security Context derived from the session.
• If EDHOC message_4 is not used, the Initiator completes a session after sending EDHOC message_3. After that, it remains generally appropriate for the Initiator to abort the EDHOC session in the event that the Initiator receives an EDHOC error message pertaining to the session. In particular, the EDHOC error message might have been legitimately sent by the Responder that failed to process EDHOC message_3. If an active adversary injects an EDHOC error message intended to the Initiator and pertaining to the EDHOC session, the Initiator would effectively receive and process that message only during a specific time interval, i.e., from when the Initiator sends EDHOC message_3 until when the Initiator frees up the CoAP Token value used in the CoAP request message that conveyed EDHOC message_3. However, the Initiator can safely set the EDHOC session to ignore any incoming EDHOC error message pertaining to the session, after successfully processing an incoming OSCORE-protected message received from the Responder. Similarly to the case previously discussed, this preserves the OSCORE Security Context derived from the session, in the event that the Initiator receives EDHOC error messages.

Following the reception and successful verification for the first time of an OSCORE-protected message using the OSCORE Security Context derived from a completed EDHOC session, a recipient EDHOC peer has different ways for setting the session to ignore pertaining EDHOC error messages from then on. Some approaches can be easier and more appealing to use than others, depending on the specific EDHOC implementation and its integration with the communication stack. As an example, two possible approaches are described below:

• The OSCORE library used or an OSCORE layer that takes part in the communication stack can be aware that an OSCORE Security Context CTX was derived from an EDHOC session S. In such a case, after receiving and successfully verifying for the first time an OSCORE-protected message using CTX, the OSCORE library/layer on the recipient EDHOC peer can effectively set the EDHOC session S to ignore any incoming EDHOC error message pertaining to the session from then on.
• When receiving an EDHOC error message pertaining to a completed EDHOC session, EDHOC can check whether the session is set to ignore pertaining EDHOC error messages. If that is the case, the received EDHOC error message is ignored. Otherwise, EDHOC checks whether the OSCORE Security Context CTX derived from the EDHOC session has been used at least once for successfully verifying an incoming OSCORE-protected message from the other EDHOC peer (e.g., by checking the Replay Window within the Recipient Context of CTX). In the case that CTX has not been used yet, the received EDHOC error message is processed as usual, i.e., the EDHOC session is aborted and CTX is deleted. Instead, in the case that CTX has been used at least once for successfully verifying an incoming OSCORE-protected message from the other EDHOC peer, the EDHOC error message is ignored and EDHOC sets the EDHOC session to ignore pertaining EDHOC error messages from then on.

Trust Policies for Learning New Authentication Credentials A peer P relies on authentication credentials associated with other peers, in order to authenticate those peers when running EDHOC with them. There are different ways for P to acquire an authentication credential CRED associated with another peer. For example, CRED can be supplied to P out-of-band by a trusted provider. Alternatively, CRED can be specified by the other peer during the EDHOC execution with P. This can rely on EDHOC message_2 or message_3, whose respective ID_CRED_R and ID_CRED_I field can specify CRED by value, or instead a URI or other external reference where CRED can be retrieved from (see ). Also during the EDHOC execution, an External Authorization Data (EAD) field might include an EAD item that specifies CRED by value, or instead a URI or other external reference where CRED can be retrieved from. This is the case, e.g., for an EAD item defined by the EDHOC and OSCORE profile of the ACE framework . In particular, the EAD item is used for transporting an access token, which in turn specifies by value or by reference the public authentication credential associated with the EDHOC peer acting as ACE client. When obtaining a new credential CRED, the peer P has to validate it before storing it. The validation steps to perform depend on the specific type of CRED (e.g., a public key certificate ) and can rely on (the authentication credential associated with) a trusted third party acting as a trust anchor. Upon retrieving a new CRED through the processing of a received EDHOC message and following the successful validation of CRED, the peer P stores CRED only if it assesses CRED to also be (provisionally) trusted, while it must not store CRED otherwise. An exception applies for the two unauthenticated operations described in , where a trust relationship with an unknown or not-yet-trusted endpoint is established later. In such a case, CRED is verified out-of-band at a later stage, or an EDHOC session key is bound to an identity out-of-band at a later stage. When processing a received EDHOC message M that specifies an authentication credential CRED, the peer P can enforce one of the trust policies LEARNING and NO-LEARNING specified in and , in order to determine whether to trust CRED. provides a summary of the behavior of P about accepting CRED, for different trust policies. Provisional trust on CRED has to be later confirmed, e.g., by information that vouches for CRED and is conveyed within an EDHOC message received during the EDHOC session (e.g., transported by an EAD item). Summary about Accepting the Authentication Credential CRED Associated with the Other Peer in an EDHOC Session, for Different Trust Policies.

	CRED is not new (already stored)	CRED is new (not already stored)
LEARNING policy	Accept, if still valid and trusted.	Accept, if valid and therefore (provisionally) trusted.
NO-LEARNING policy, without acceptable exception	Accept, if still valid and trusted.	Do not accept.
NO-LEARNING policy, with acceptable exception	Accept, if still valid and trusted.	Accept, if valid and (provisionally) trusted.

Irrespective of the adopted trust policy, P actually uses CRED only if it is determined to be fine to use in the context of the ongoing EDHOC session, also depending on the specific identity of the other peer (see Sections and of ). If this is not the case, P aborts the EDHOC session with the other peer. If P stores CRED, then P will consider CRED as valid and trusted until:

• CRED possibly becomes invalid, e.g., because it expires or because P gains knowledge that it has been revoked; or
• CRED becomes non-trusted, e.g., because P originally assessed CRED to be provisionally trusted, and later on failed to obtain an expected final confirmation of trust.

P must delete CRED from its local storage if CRED becomes invalid or non-trusted. When storing CRED, the peer P should generate the authentication credential identifier(s) corresponding to CRED and store them as associated with CRED. For example, if CRED is a public key certificate, an identifier of CRED can be the hash of the certificate. In general, P should generate and associate with CRED one corresponding identifier for each type of authentication credential identifier that P supports and that is compatible with CRED. In future executions of EDHOC with the other peer associated with CRED, this allows such other peer to specify CRED by reference, e.g., by indicating its credential identifier as ID_CRED_R/ID_CRED_I in the EDHOC message_2 or message_3 addressed to the peer P. In turn, this allows P to retrieve CRED from its local storage.

Trust Policy LEARNING When enforcing the LEARNING policy, the peer P trusts CRED even if P is not already storing CRED at message reception time. That is, upon receiving M, the peer P performs the following steps.

1. P retrieves CRED, as specified by reference or by value in the ID_CRED_I/ID_CRED_R field of M or in the value of an EAD item of M.
2. P checks whether CRED is already being stored and if it is still valid and trusted. In such a case, P trusts CRED and can continue the EDHOC execution. Otherwise, P moves to Step 3.
3. P attempts to validate CRED. If the validation process is not successful, P aborts the EDHOC session with the other peer. Otherwise, P trusts and stores CRED, and can continue the EDHOC execution.

Trust Policy NO-LEARNING When enforcing the NO-LEARNING policy, the peer P trusts CRED only if P is already storing CRED at message reception time, unless in cases where situation-specific exceptions apply and are deliberately enforced (see below). That is, upon receiving M, the peer P continues the execution of EDHOC only if both the following conditions hold:

• P currently stores CRED, as specified by reference or by value in the ID_CRED_I/ID_CRED_R field of M or in the value of an EAD item of M; and
• CRED is still valid (i.e., P believes CRED to not be expired or revoked) and trusted.

Exceptions may apply and be actually enforced in cases where, during an EDHOC execution, P obtains additional information that allows it to trust and successfully validate CRED, even though P was not already storing CRED when receiving M. Such exceptions typically rely on a trusted party that vouches for CRED, e.g., in the cases discussed in . From the point of view of the peer P, the trusted party might have been involved in the background, so that the vouching information about CRED is conveyed within an EDHOC message received during the EDHOC session (e.g., transported by an EAD item). Alternatively, P might directly interact with the trusted party for retrieving the vouching information about CRED, e.g., after having received M and before continuing the EDHOC execution. If the peer P admits such an exception and actually enforces it on an authentication credential CRED, then P effectively handles CRED according to the trust policy "LEARNING" specified in . When doing so, P still attempts to validate CRED, and it aborts the EDHOC session if the validation process is not successful.

Enforcement of Trust Policies in Specific Scenarios The following subsections discuss how an EDHOC peer enforces the trust policies LEARNING and NO-LEARNING in specific scenarios.

In the EDHOC and OSCORE Profile of ACE As discussed in , two EDHOC peers can be using the ACE framework and specifically the EDHOC and OSCORE profile of ACE defined in . In this case, one of the two EDHOC peers, namely PEER_RS, acts as ACE resource server (RS). Instead, the other EDHOC peer, namely PEER_C, acts as ACE client and obtains from an ACE authorization server (AS) an access token for accessing protected resources at PEER_RS. The AS and PEER_RS are in a trust relationship. Together with other information, the access token specifies (by value or by reference) the public authentication credential AUTH_CRED_C associated with PEER_C that PEER_C is going to use when running EDHOC with PEER_RS. Note that AUTH_CRED_C will be used as either CRED_I or CRED_R in EDHOC, depending on whether the two peers use the EDHOC forward message flow (i.e., PEER_C is the EDHOC Initiator) or the EDHOC reverse message flow (i.e., PEER_C is the EDHOC Responder), respectively (see ). When the AS issues the first access token that specifies AUTH_CRED_C and is intended to be uploaded to PEER_RS, it is expected that the access token specifies AUTH_CRED_C by value and that PEER_RS is not currently storing AUTH_CRED_C, but instead will obtain it and learn it upon receiving the access token. Although the AS can upload the access token to PEER_RS on behalf of PEER_C as per the alternative SDC workflow defined in , the access token is typically uploaded to PEER_RS by PEER_C through a dedicated EAD item, when running EDHOC with PEER_RS. The specific EDHOC message that includes the EAD item conveying the access token depends on whether the two peers use the EDHOC forward message flow or the EDHOC reverse message flow. Consequently, PEER_RS has to learn AUTH_CRED_C as a new authentication credential during an EDHOC session with PEER_C. At least for its EDHOC resource used for exchanging the EDHOC messages of the EDHOC session in question, this requires PEER_RS to:

• Enforce the trust policy "LEARNING"; or
• If enforcing the trust policy "NO-LEARNING", additionally enforce an overriding exception when an incoming EDHOC message includes an EAD item specifying a valid access token issued by a trusted AS. That is, through a successful verification of the access token, PEER_RS is able to trust AUTH_CRED_C (if found valid), even though it was not already storing AUTH_CRED_C when receiving the EDHOC message with the EAD item.

In the Lightweight Authorization using EDHOC (ELA) Procedure When the execution of EDHOC embeds the ELA procedure defined in , the EDHOC peer U receives an EDHOC message_2 (message_3) where ID_CRED_R (ID_CRED_I) specifies by value the authentication credential CRED associated with the other peer V. Furthermore, an EDHOC message sent to U includes an EAD item, which specifies a voucher issued by a trusted enrollment server W. The voucher is an assertion to U that W has authorized V and has endorsed CRED. The specific EDHOC message that includes the EAD item conveying the voucher depends on whether U and V use the EDHOC forward message flow (i.e., U is the EDHOC Initiator) or the EDHOC reverse message flow (i.e., U is the EDHOC Responder). In particular:

• When using the EDHOC forward message flow, the EAD item is included in EDHOC message_4, thereby endorsing CRED that was specified by ID_CRED_R in the previous EDHOC message_2. Since it is indeed expected that U is not already storing CRED upon receiving EDHOC message_2, U can at best provisionally trust CRED (if found valid) when retrieving it from EDHOC message_2.
• When using the EDHOC reverse message flow, the EAD item is included in EDHOC message_3, thereby endorsing CRED that is specified by ID_CRED_I in the same EDHOC message.

In either case, through a successful verification of the voucher, U is able to ultimately trust CRED (if found valid), even though U was not already storing CRED upon receiving the EDHOC message specifying CRED. Therefore, if U enforces the trust policy "NO-LEARNING", it can additionally enforce an overriding exception as below:

• When using the EDHOC forward message flow, the exception is enforced when processing an EDHOC message_2, and it is raised by the intention of using the ELA procedure. That is, U intends to proceed with sending a consistent EDHOC message_3 that indicates the wish to obtain a voucher issued by W. At that point in time, the authentication credential CRED is only provisionally trusted (if found valid), with the expectation to receive an EDHOC message_4 in the same EDHOC session, conveying a valid voucher issued by W and thus confirming that CRED can be ultimately trusted.
• When using the EDHOC reverse message flow, the exception is enforced when processing an EDHOC message_3, and it is raised by the EDHOC message_3 including an EAD item that conveys a valid voucher issued by W, thus confirming that CRED can be ultimately trusted.

Side Processing of Incoming EDHOC Messages This section describes an approach that EDHOC peers can use upon receiving EDHOC messages, in order to fetch/validate authentication credentials and to process External Authorization Data (EAD) items. The transport mechanism provided by EDHOC for conveying EAD items is defined in . In particular, an EDHOC message_x can include one dedicated EAD field EAD_x, for x = 1, 2, 3, or 4. In turn, an EAD field can include one or more EAD items. As per , specifications defining those EAD items have to set the ground for "agreeing on the surrounding context and the meaning of the information passed to and from the application". The approach described in this section aims to help implementers navigate the surrounding context mentioned above, irrespective of the specific EAD items conveyed in the EDHOC messages. In particular, the described approach takes into account the following points:

• The fetching and validation of the authentication credential associated with the other peer relies on ID_CRED_I in EDHOC message_2, or on ID_CRED_R in EDHOC message_3, or on the value of an EAD item. When this occurs upon receiving EDHOC message_2 or message_3, the decryption of the EDHOC message has to be completed first. Validating the authentication credential or assessing whether it is trusted might depend on using the value of an EAD item, which in turn has to be validated first.
• It is possible that some EAD items can be processed only after having successfully verified the EDHOC message, i.e., after a successful verification of the Signature_or_MAC field in EDHOC message_2 or message_3. For instance, an EAD item within the EAD_3 field of EDHOC message_3 might contain a Certificate Signing Request (CSR) . Hence, such an EAD item can be processed only once the recipient peer has attained proof that the other peer possesses its own private key.

In order to conveniently handle such processing, the application can prepare in advance a "side-processor object" (SPO), which takes care of the operations above during the EDHOC execution. In particular, the application provides EDHOC with the SPO before starting an EDHOC execution, during which EDHOC will temporarily transfer control to the SPO at the right point in time, in order to perform the required side-processing of an incoming EDHOC message. Furthermore, the application has to instruct the SPO about:

• How to prepare any EAD item such that: it has to be included in the EAD field of an outgoing EDHOC message, possibly together with other EAD items; and it is independent of the processing of other EAD items included in incoming EDHOC messages. This includes, for instance, the preparation of padding EAD items (see ).
• The list of one or more EAD items that are expected to be present within the dedicated EAD field of specific, incoming EDHOC messages during the EDHOC session. This takes into account, for instance, external security applications that will be integrated in the EDHOC session (e.g., see ). Throughout the EDHOC session, the SPO keeps such a list of expected EAD items up-to-date. This takes into account, for instance, external security applications that have been run integrated in the EDHOC session, the current status of the session, as well as the EDHOC messages that have been exchanged during the session and the outcome of their processing.

At the right point in time during the processing of an incoming EDHOC message M at the peer P, EDHOC invokes the SPO and provides it with the following input:

• When M is EDHOC message_2 or message_3, an indication of whether this invocation is happening before or after the message verification (i.e., before or after having verified the Signature_or_MAC field).
• The full set of information related to the current EDHOC session. This especially includes the selected cipher suite and the ephemeral Diffie-Hellman public keys G_X and G_Y that the two peers have exchanged in the EDHOC session.
• The authentication credentials that the peer P stores as associated with other peers.
• All the decrypted information elements retrieved from M.
• The EAD items included in M.
  • Note that EDHOC might do some preliminary work on M before invoking the SPO, in order to provide the SPO only with actually relevant EAD items. This requires the application to additionally provide EDHOC with the ead_labels of the EAD items that the peer P recognizes (see ). With such information available, EDHOC can early abort the current session if M includes any EAD item which is both critical and not recognized by the peer P. If no such EAD items are found, EDHOC can remove any padding EAD item (see ), as well as any EAD item which is neither critical nor recognized (since the SPO is going to ignore it anyway). This results in EDHOC providing the SPO only with EAD items that will be recognized and that require actual processing.
  • Note that, after having processed the EAD items, the SPO might actually need to store them throughout the whole EDHOC execution, e.g., in order to refer to them also when processing later EDHOC messages in the current EDHOC session.

The SPO performs the following tasks on an incoming EDHOC message M:

• The SPO checks whether M does not include an EAD item whose presence was expected, based on the related list maintained throughout the EDHOC session. If such an EAD item is absent, the SPO can come to an early determination about whether and how to proceed with the processing of M. In particular, if an EAD item is absent although its presence was strictly required, then the SPO can early abort the EDHOC session, thereby avoiding potentially costly operations (e.g., the retrieval and validation of the authentication credential associated with the other peer).
• The SPO fetches and/or validates the authentication credential CRED associated with the other peer, based on a dedicated EAD item of M or on the ID_CRED field of M (for EDHOC message_2 or message_3).
• The SPO processes the EAD items conveyed in the EAD field of M.
• The SPO stores the results of the performed operations and makes such results available to the application.
• When the SPO has completed its side processing and transfers control back to EDHOC, the SPO provides EDHOC with the produced EAD items to include in the EAD field of the next outgoing EDHOC message. The production of such EAD items can be triggered, e.g., by:
  • The consumption of EAD items included in M.
  • The execution of instructions that the SPO has received from the application, concerning EAD items to produce irrespective of other EAD items included in M.

In the following, to describe more in detail the actions performed by the SPO on the different incoming EDHOC messages. Then, describes further special handling of incoming EDHOC messages, as to consistency checks concerning authentication credentials in particular situations. After completing the EDHOC execution, control is transferred back to the application. In particular, the application is provided with the overall outcome of the EDHOC execution (i.e., successful completion or failure), together with possible specific results produced by the SPO throughout the EDHOC execution (e.g., due to the processing of EAD items). After that, the application might need to perform follow-up actions, depending on the outcome of the EDHOC execution. For example, the SPO might have preliminarily filled application-level data structures, as a result of processing EAD items. In the case of a successful EDHOC execution, the application might need to finalize such data structures. Instead, in the case of an unsuccessful EDHOC execution, the application might need to clean-up or amend such data structures, or even roll back what the SPO did, unless the SPO already performed such actions before control was transferred back to the application.

EDHOC message_1 During the processing of an incoming EDHOC message_1, EDHOC invokes the SPO only once, after the Responder peer has successfully decoded the message and accepted the selected cipher suite. If the EAD_1 field is present, the SPO processes the EAD items included therein. Once all such EAD items have been processed, the SPO transfers control back to EDHOC. When doing so, the SPO also provides EDHOC with any produced EAD items to include in the EAD field of the next outgoing EDHOC message. Then, EDHOC resumes its execution and advances its protocol state. Future extensions of EDHOC or external security applications integrated into EDHOC might require a processing of EDHOC message_1 that is more advanced than the currently expected one. In particular, an EAD item conveyed in EDHOC message_1 might specify the authentication credential CRED associated with the Initiator (by value or by reference), as wrapped in a cryptographically protected "envelope". In such a case, the processing of an incoming EDHOC message_1 shares similarities with that of an incoming EDHOC message_2 or message_3 (see ), as it is further elaborated in .

EDHOC message_4 During the processing of an incoming EDHOC message_4, EDHOC invokes the SPO only once, after the Initiator peer has successfully decrypted the message. If the EAD_4 field is present, the SPO processes the EAD items included therein. Once all such EAD items have been processed, the SPO transfers control back to EDHOC, which resumes its execution and advances its protocol state.

EDHOC message_2 and message_3 The following refers to "message_X" as an incoming EDHOC message_2 or message_3, and to "message verification" as the verification of Signature_or_MAC_X in message_X. During the processing of a message_X, EDHOC invokes the SPO two times:

• Right after message_X has been decrypted and before its verification starts. Following this invocation, the SPO performs the actions described in .
• Right after message_X has been successfully verified. Following this invocation, the SPO performs the actions described in .

The flowcharts in show the high-level interaction between the core EDHOC processing and the SPO, as well as the different steps taken for processing an incoming message_X.

Pre-Verification Side Processing The pre-verification side processing occurs in two sequential phases, namely PHASE_1 (see ) and PHASE_2 (see ).

PHASE_1 During PHASE_1, the SPO at the recipient peer P determines CRED, i.e., the authentication credential associated with the other peer to be used in the ongoing EDHOC session. In particular, the SPO first checks whether expected EAD items are absent in message_X (see ), and then performs the following steps.

1. The SPO determines CRED based on ID_CRED_X or on an EAD item included in message_X. Those may specify CRED by value or by reference, including a URI or other external reference where CRED can be retrieved from. If CRED is already stored, the SPO moves to Step 2. Otherwise, the SPO moves to Step 3.
2. The SPO determines if the stored CRED is currently trusted and valid, e.g., by verifying that CRED has not expired and has not been revoked. Performing such a validation might require the SPO to first process an EAD item included in message_X. For example, it can be an EAD item in EDHOC message_2 that confirms or revokes the validity of CRED_R specified by ID_CRED_R, as the result of an OCSP process . In the case that CRED is determined to be valid, the SPO moves to Step 9. Otherwise, the SPO moves to Step 11.
3. The SPO attempts to retrieve CRED via ID_CRED_X or an EAD item considered at Step 1. Then, the SPO moves to Step 4.
4. If the retrieval of CRED has succeeded, the SPO moves to Step 5. Otherwise, the SPO moves to Step 11.
5. If the enforced trust policy for new authentication credentials is "NO-LEARNING" and P does not admit any exceptions that are acceptable to enforce for message_X (see ), the SPO moves to Step 11. Otherwise, the SPO moves to Step 6.
6. If this step has been reached, the peer P is not already storing the retrieved CRED and, at the same time, it enforces either the trust policy "LEARNING" or the trust policy "NO-LEARNING" while also enforcing an exception acceptable for message_X (see ). Consistent with that, the SPO determines if CRED is currently valid, e.g., by verifying that CRED has not expired and has not been revoked. Validating CRED might require the SPO to first process an EAD item included in message_X. For example, it can be an OCSP response for validating CRED_R as a public key certificate transported by value or reference in ID_CRED_R. After successfully validating CRED, the peer P can typically consider CRED as ultimately trusted as well. However, there can be cases where P requires to obtain additional information before doing so. If such additional information can be retrieved from message_X (e.g., from an EAD item included therein), then P uses it to assess if CRED is trusted. Otherwise, if such additional information is expected later on during the EDHOC session, it can be acceptable for P to consider CRED as provisionally trusted. discusses the ELA procedure defined in , as a case in point where additional information required by the peer P to trust CRED could not be included in the same message that specifies CRED. After completing the validation and trust assessment of CRED, the SPO moves to Step 7.
7. If CRED has been determined valid and (provisionally) trusted, the SPO moves to Step 8. Otherwise, the SPO moves to Step 11.
8. The SPO stores CRED as a valid and (provisionally) trusted authentication credential associated with the other peer, together with corresponding authentication credential identifiers (see ). Then, the SPO moves to Step 9.
9. The SPO checks if CRED is fine to use in the context of the ongoing EDHOC session, also depending on the specific identity of the other peer (see Sections and of ). If this is the case, the SPO moves to Step 10. Otherwise, the SPO moves to Step 11.
10. P uses CRED as authentication credential associated with the other peer in the ongoing EDHOC session. Then, PHASE_1 ends and the pre-verification side processing moves to the next PHASE_2 (see ).
11. The SPO has not found a valid and (provisionally) trusted authentication credential associated with the other peer that can be used in the ongoing EDHOC session. Therefore, the EDHOC session with the other peer is aborted.

PHASE_2 During PHASE_2, the SPO processes any EAD item included in message_X such that both the following conditions hold:

• The EAD item has not already been processed during PHASE_1.
• The EAD item can be processed before performing the verification of message_X.

Once all such EAD items have been processed, the SPO transfers control back to EDHOC, which either aborts the ongoing EDHOC session or continues the processing of message_X with its corresponding message verification.

Post-Verification Side Processing During the post-verification side processing, the SPO processes any EAD item included in message_X such that the processing of that EAD item had to wait for completing the successful message verification. The late processing of such EAD items is typically due to the fact that a pre-requirement has to be fulfilled first. For example, the recipient peer P has to have first verified that the other peer does possess the private key corresponding to the public key specified by CRED, i.e., the authentication credential associated with the other peer that was determined during the pre-verification side processing (see ). This requirement is fulfilled after a successful verification of message_X. Once all such EAD items have been processed, the SPO transfers control back to EDHOC. When doing so, the SPO also provides EDHOC with any produced EAD items to include in the EAD field of the next outgoing EDHOC message. Then, EDHOC resumes its execution and advances its protocol state.

Flowcharts The flowchart in shows the high-level interaction between the core EDHOC processing and the SPO, with particular reference to an incoming EDHOC message_2 or message_3.

High-Level Interaction Between the Core EDHOC Processing and the Side-Processor Object (SPO), for Incoming EDHOC message_2 and message_3. | Retrieve the | | Advance the | | | | message_X | | protocol state | | protocol state | | | +-----------+ +----------------+ +----------------+ | | | ^ | | | | | | v | | | +--------------+ +--------------------+ | | | | Decrypt | | Verify | | | | | CIPHERTEXT_X | | Signature_or_MAC_X | | | | +--------------+ +--------------------+ | | | | ^ | | | | | | | | | +----------------|-----------|-----------|---------|------------------+ | | | | | | | | ................. Divert | Get | Divert | Get | : EAD items : | back | | back | : for the next : | | | | : EDHOC message : | | | | :...............: | | | | +----------------|-----------|-----------|---------|------------------+ | | | | | | | v | v | | | +---------------------------+ +-----------------------------+ | | | a) Check whether expected | | Processing of | | | | EAD items are absent | | post-verification EAD items | | | | b) Retrieval and | +-----------------------o-----+ | | | validation of CRED_X; | | | | | c) Trust assessment o-------- Shared state -------o | | | of CRED_X; | | | | d) Processing of | ...................... | | | pre-verification | : Instructions about : | | | EAD items | : EAD items to : | | | | : unconditionally : | | | - (b) and (d) might have | : produce for the : | | | to occur in parallel | : next EDHOC message : | | | - (c) depends on the | :....................: | | | trust policy used | | | +---------------------------+ | | Side-Processor Object (SPO) | +---------------------------------------------------------------------+ ]]>

The flowchart in shows the different steps taken for processing an incoming EDHOC message_2 and message_3.

Processing Steps for Incoming EDHOC message_2 and message_3. | CRED via |---->| retrieval | | | | point to an already | | ID_CRED_X or | | of CRED | | | | stored CRED? | | an EAD item | | successful? | | | +---------------------+ +--------------+ +-------------+ | | | | | | | | | NO | YES | | | +----------------+ | | | | | | | | | YES | | | | v v v | | +-----------------+ NO +-----------+ YES +-----------------+ | | | 2. Is this CRED |-------->| 11. Abort |<------| 5. Is the trust | | | | still valid and | | the EDHOC | | policy used | | | | trusted? | | session | | "NO-LEARNING", | | | +-----------------+ | | | without any | | | | | | | acceptable | | | | | | | exceptions? | | | | | | +-----------------+ | | | YES | | | | | v | | Here the trust | NO | | +--------------------+ NO | | policy used is | | | | 9. Is this CRED |----->| | "LEARNING", or | | | | good to use in the | +-----------+ "NO-LEARNING" | | | | context of this | ^ together with | | | | EDHOC session? |<-----+ | an overriding | | | +--------------------+ | | exception | | | | | | | | | | | | v | | | | | +---------------+ | | | | | | 6. Assess if | | | | | | | CRED is valid | | | | | | | and trusted | | | | | | +---------------+ | | | | | | | | | YES | | NO | | | | | | v | | | | +-------------------------------+ | | | | | 7. Is CRED valid and | | | | | | (provisionally) trusted? | | | | | +-------------------------------+ | | | | | | | | | | YES | | v | v | | +------------------+ | +-------------------------------+ | | | 10. Continue by | | | 8. Store CRED as valid and | | | | considering this | +-----| (provisionally) trusted. | | | | CRED as the | | | | | | authentication | | Pair CRED with consistent | | | | credential | | credential identifiers, for | | | | associated with | | each supported type of | | | | the other peer | | credential identifier. | | | +------------------+ +-------------------------------+ | | | | +---------|-----------------------------------------------------------+ | | +---------|-----------------------------------------------------------+ | | Pre-verification side processing (PHASE_2) | | v | | +--------------------------------------------------------+ | | | Process the EAD items that have not been processed yet | | | | and that can be processed before message verification | | | +--------------------------------------------------------+ | | | | +---------|-----------------------------------------------------------+ | | v Control transferred back to the core EDHOC processing | | v +------------------+ | Verify message_X | (Core EDHOC processing) +------------------+ | | v Control transferred to the side-processor object | +---------|----------------------------------------+ | | Post-verification processing | | v | | +---------------------------------------------+ | | | Process the EAD items that have to be | | | | processed (also) after message verification | | | +---------------------------------------------+ | | | | | | | | v | | +--------------------------------------------+ | | | Make all the results of the EAD processing | | | | available to build the next EDHOC message | | | +--------------------------------------------+ | | | | +---------|----------------------------------------+ | | v Control transferred back to the core EDHOC processing | | v +----------------+ | Advance the | (Core EDHOC processing) | protocol state | +----------------+ ]]>

Consistency Checks of Authentication Credentials from ID_CRED and EAD Items Typically, an EDHOC peer specifies its associated authentication credential (by value or by reference) only in the ID_CRED field of EDHOC message_2 (if acting as Responder) or EDHOC message_3 (if acting as Initiator). In addition to that, there may be cases where an EDHOC peer provides the authentication credential also in an EAD item. In particular, such an EAD item can specify a cryptographically protected "envelope" (by value or by reference), which in turn specifies the authentication credential (by value or by reference). A case in point is the EDHOC and OSCORE profile of the ACE framework , where the envelope in question is an access token issued to the ACE client. In such a case, the ACE client can rely on an EAD item specifying the access token, which in turn specifies the authentication credential (by value or by reference) associated with the client. During an EDHOC session, an EDHOC peer P1 might therefore receive the authentication credential CRED associated with the other EDHOC peer P2 as specified by two items:

• ITEM_A: the ID_CRED field specifying CRED. If P2 acts as Initiator (Responder), then ITEM_A is the ID_CRED_I (ID_CRED_R) field.
• ITEM_B: the envelope specified in an EAD item within an EDHOC message sent by P2.

As part of the process where P1 validates CRED during the EDHOC session, P1 must check that both ITEM_A and ITEM_B specify the same authentication credential, and it must abort the EDHOC session if such a consistency check fails. The consistency check is successful only if one of the following conditions holds, and it fails otherwise:

• If both ITEM_A and ITEM_B specify an authentication credential by value, then both ITEM_A and ITEM_B specify the same value.
• If one among ITEM_A and ITEM_B specifies an authentication credential by value VALUE while the other one specifies an authentication credential by reference REF, then REF is a valid reference for VALUE.
• If ITEM_A specifies an authentication credential by reference REF_A and ITEM_B specifies an authentication credential by reference REF_B, then REF_A or REF_B allows to retrieving the value VALUE of an authentication credential from a local or remote storage, such that both REF_A and REF_B are a valid reference for VALUE.

The peer P1 performs the consistency check above as soon as it has both ITEM_A and ITEM_B available. If P1 acts as Responder, that is the case when processing the incoming EDHOC message_3. If P1 acts as Initiator, that is the case when processing the incoming EDHOC message_2 or message_4, i.e., whichever of the two messages includes ITEM_B in an EAD item of its EAD field.

Using EDHOC over CoAP with Block-Wise specifies how to transfer EDHOC over CoAP . In such a case, EDHOC messages (possibly prepended by an EDHOC connection identifier) are transported in the payload of CoAP requests and responses, according to the EDHOC forward message flow or the EDHOC reverse message flow. Furthermore, specifies how to derive an OSCORE Security Context from an EDHOC session. Building on that, further details the use of EDHOC with CoAP and OSCORE. In particular, it specifies an optimization approach for the EDHOC forward message flow that combines the EDHOC execution with the first subsequent OSCORE transaction. This is achieved by means of an "EDHOC + OSCORE request", i.e., a single CoAP request message that conveys both EDHOC message_3 of the ongoing EDHOC session and the OSCORE-protected application data, where the latter is protected with the OSCORE Security Context derived from that EDHOC session. This section provides guidelines and recommendations for CoAP endpoints supporting Block-wise transfers for CoAP and specifically for CoAP clients supporting the EDHOC + OSCORE request defined in . The use of Block-wise transfers can be desirable, e.g., for EDHOC messages that include a large ID_CRED_I or ID_CRED_R, or that include a large EAD field. The following especially considers a CoAP endpoint that may perform only "inner" Block-wise, but not "outer" Block-wise operations (see ). That is, the considered CoAP endpoint does not (further) split an OSCORE-protected message like an intermediary (e.g., a proxy) might do. This is the typical case for CoAP endpoints using OSCORE (see ).

Notation The rest of this section refers to the following notation:

• SIZE_BODY: the size in bytes of the data to be transmitted with CoAP. When Block-wise is used, such data is referred to as the "body" to be fragmented into blocks, each of which to be transmitted in one CoAP message. With the exception pertaining to EDHOC message_3 described in the following paragraph, the considered body can in general be an EDHOC message, possibly prepended by an EDHOC connection identifier encoded as per . When specifically using the EDHOC + OSCORE request, the considered body is the application data to be protected with OSCORE, (whose first block is) to be sent together with EDHOC message_3 as part of the EDHOC + OSCORE request.
• SIZE_EDHOC_M3: the size in bytes of EDHOC message_3, if this is sent as part of the EDHOC + OSCORE request. Otherwise, the size in bytes of EDHOC message_3, plus, if included, the size in bytes of a prepended EDHOC connection identifier encoded as per .
• SIZE_MTU: the maximum amount of transmittable bytes before having to use Block-wise. This is, for example, 64 KiB as maximum datagram size when using UDP, or 1280 bytes as the maximum size for an IPv6 MTU.
• SIZE_OH: the size in bytes of the overall overhead due to all the communication layers underlying the application. This takes into account also the overhead introduced by the OSCORE processing.
• LIMIT = (SIZE_MTU - SIZE_OH): the practical maximum size in bytes to be considered by the application before using Block-wise.
• SIZE_BLOCK: the size in bytes of inner blocks.
• ceil(): the ceiling function.

Pre-requirements for the EDHOC + OSCORE Request Before sending an EDHOC + OSCORE request, a CoAP client has to perform the following checks. Note that, while the CoAP client is able to fragment the plain application data before any OSCORE processing, it cannot fragment the EDHOC + OSCORE request or the EDHOC message_3 added therein.

• If inner Block-wise is not used, hence SIZE_BODY <= LIMIT, the CoAP client must verify whether all the following conditions hold:
  • COND1: SIZE_EDHOC_M3 <= LIMIT
  • COND2: (SIZE_BODY + SIZE_EDHOC_M3) <= LIMIT
• If inner Block-wise is used, the CoAP client must verify whether all the following conditions hold:
  • COND3: SIZE_EDHOC_M3 <= LIMIT
  • COND4: (SIZE_BLOCK + SIZE_EDHOC_M3) <= LIMIT

In either case, if not all the corresponding conditions hold, the CoAP client should not send the EDHOC + OSCORE request. Instead, the CoAP client can continue by switching to the purely sequential, original EDHOC workflow (see ). That is, the CoAP client first sends EDHOC message_3 prepended by the EDHOC Connection Identifier C_R encoded as per and then sends the OSCORE-protected CoAP request once the EDHOC execution is completed.

Effectively Using Block-Wise In order to avoid further fragmentation at lower layers when sending an EDHOC + OSCORE request, the CoAP client has to use inner Block-wise if any of the following conditions holds:

• COND5: SIZE_BODY > LIMIT
• COND6: (SIZE_BODY + SIZE_EDHOC_M3) > LIMIT

In particular, consistent with , the SIZE_BLOCK used has to be such that the following condition also holds:

• COND7: (SIZE_BLOCK + SIZE_EDHOC_M3) <= LIMIT

Note that the CoAP client might still use Block-wise due to reasons different from exceeding the size indicated by LIMIT. The following shows the number of round trips for completing both the EDHOC execution and the first OSCORE-protected exchange, under the assumption that the exchange of EDHOC message_1 and EDHOC message_2 does not result in using Block-wise. If both the conditions COND5 and COND6 hold, the use of Block-wise results in the following number of round trips experienced by the CoAP client.

• If the original EDHOC execution workflow is used (see ), the number of round trips RT_ORIG is equal to 1 + ceil(SIZE_EDHOC_M3 / SIZE_BLOCK) + ceil(SIZE_BODY / SIZE_BLOCK).
• If the optimized EDHOC execution workflow is used (see ), the number of round trips RT_COMB is equal to 1 + ceil(SIZE_BODY / SIZE_BLOCK).

It follows that RT_COMB < RT_ORIG, i.e., the optimized EDHOC execution workflow always yields a lower number of round trips. Instead, the convenience of using the optimized EDHOC execution workflow becomes questionable if both the following conditions hold:

• COND8: SIZE_BODY <= LIMIT
• COND9: (SIZE_BODY + SIZE_EDHOC_M3) > LIMIT

That is, since SIZE_BODY <= LIMIT, using Block-wise would not be required when using the original EDHOC execution workflow, provided that SIZE_EDHOC_M3 <= LIMIT still holds. At the same time, using the combined workflow is in itself what actually triggers the use of Block-wise, since (SIZE_BODY + SIZE_EDHOC_M3) > LIMIT. Therefore, the following round trips are experienced by the CoAP client.

• The original EDHOC execution workflow run without using Block-wise results in a number of round trips RT_ORIG equal to 3.
• The optimized EDHOC execution workflow run using Block-wise results in a number of round trips RT_COMB equal to 1 + ceil(SIZE_BODY / SIZE_BLOCK).

It follows that RT_COMB >= RT_ORIG, i.e., the optimized EDHOC execution workflow might still be not worse than the original EDHOC execution workflow in terms of round trips. This is the case only if the SIZE_BLOCK used is such that ceil(SIZE_BODY / SIZE_BLOCK) is equal to 2, i.e., the plain application data is fragmented into only 2 inner blocks, and thus the EDHOC + OSCORE request is followed by only one more request message transporting the last block of the body. However, even in such a case, there would be no advantage in terms of round trips compared to the original workflow, while still requiring the CoAP client and the CoAP server to perform the processing due to using the EDHOC + OSCORE request and Block-wise transferring. Therefore, if both the conditions COND8 and COND9 hold, the CoAP client should not send the EDHOC + OSCORE request. Instead, the CoAP client should continue by switching to the original EDHOC execution workflow. That is, the CoAP client first sends EDHOC message_3 prepended by the EDHOC Connection Identifier C_R encoded as per and then sends the OSCORE-protected CoAP request once the EDHOC execution is completed.

Operational Considerations There are no new operations or manageability requirements introduced by this document, which provides considerations for implementers of the EDHOC protocol and does not update the protocol or introduce extensions thereof.

Security Considerations This document provides considerations for implementations of the EDHOC protocol. The security considerations compiled in and in apply. The compliance requirements for implementations that are listed in also apply. It is foreseeable that the EDHOC protocol will be extended (e.g., in terms of new cipher suites, new methods, and new types of authentication credentials) and that external security applications will be integrated into EDHOC by embedding the transport of their data in EDHOC EAD items. For implementations that support such extensions and external applications, the related security considerations and compliance requirements also apply.

Assessing the Correctness of Implementations Tools relying on fuzz testing such as EDHOC-Fuzzer can help assess the correctness of implementations of the EDHOC protocol and of external security applications integrated into EDHOC. Such tools help finding and amending implementation errors especially related to the following points:

• Non-conformance with the protocol specification (e.g., unintended deviations in performing the protocol steps), which can be a potential source of security vulnerabilities in addition to performance deficiencies.
• Presence of inappropriate states and state transitions in the modeling of the EDHOC execution, e.g., states that are impossible to reach and traverse or that are not part of the protocol specification (which is a particular case of non-conformance). These states and transitions should be amended or removed, in order to reduce the memory footprint and code complexity and to simplify the implementation, thus reducing the risks of bugs and related security vulnerabilities.

IANA Considerations This document has no actions for IANA.

References Normative References The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. The nodes often have 8-bit microcontrollers with small amounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) often have high packet error rates and a typical throughput of 10s of kbit/s. The protocol is designed for machine- to-machine (M2M) applications such as smart energy and building automation. CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. CoAP is designed to easily interface with HTTP for integration with the Web while meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments. The Constrained Application Protocol (CoAP) is a RESTful transfer protocol for constrained nodes and networks. Basic CoAP messages work well for small payloads from sensors and actuators; however, applications will need to transfer larger payloads occasionally -- for instance, for firmware updates. In contrast to HTTP, where TCP does the grunt work of segmenting and resequencing, CoAP is based on datagram transports such as UDP or Datagram Transport Layer Security (DTLS). These transports only offer fragmentation, which is even more problematic in constrained nodes and networks, limiting the maximum size of resource representations that can practically be transferred. Instead of relying on IP fragmentation, this specification extends basic CoAP with a pair of "Block" options for transferring multiple blocks of information from a resource representation in multiple request-response pairs. In many important cases, the Block options enable a server to be truly stateless: the server can handle each block transfer separately, with no need for a connection setup or other server-side memory of previous block transfers. Essentially, the Block options provide a minimal way to transfer larger representations in a block-wise fashion. A CoAP implementation that does not support these options generally is limited in the size of the representations that can be exchanged, so there is an expectation that the Block options will be widely used in CoAP implementations. Therefore, this specification updates RFC 7252. This document defines Object Security for Constrained RESTful Environments (OSCORE), a method for application-layer protection of the Constrained Application Protocol (CoAP), using CBOR Object Signing and Encryption (COSE). OSCORE provides end-to-end protection between endpoints communicating using CoAP or CoAP-mappable HTTP. OSCORE is designed for constrained nodes and networks supporting a range of proxy operations, including translation between different transport protocols. Although an optional functionality of CoAP, OSCORE alters CoAP options processing and IANA registration. Therefore, this document updates RFC 7252. This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a very compact and lightweight authenticated Diffie-Hellman key exchange with ephemeral keys. EDHOC provides mutual authentication, forward secrecy, and identity protection. EDHOC is intended for usage in constrained scenarios, and a main use case is to establish an Object Security for Constrained RESTful Environments (OSCORE) security context. By reusing CBOR Object Signing and Encryption (COSE) for cryptography, Concise Binary Object Representation (CBOR) for encoding, and Constrained Application Protocol (CoAP) for transport, the additional code size can be kept very low. The lightweight authenticated key exchange protocol Ephemeral Diffie-Hellman Over COSE (EDHOC) can be run over the Constrained Application Protocol (CoAP) and used by two peers to establish a Security Context for the security protocol Object Security for Constrained RESTful Environments (OSCORE). This document details this use of the EDHOC protocol by specifying a number of additional and optional mechanisms, including an optimization approach for combining the execution of EDHOC with the first OSCORE transaction. This combination reduces the number of round trips required to set up an OSCORE Security Context and to complete an OSCORE transaction using that Security Context. Informative References This memo represents a republication of PKCS #10 v1.7 from RSA Laboratories' Public-Key Cryptography Standards (PKCS) series, and change control is retained within the PKCS process. The body of this document, except for the security considerations section, is taken directly from the PKCS #9 v2.0 or the PKCS #10 v1.7 document. This memo provides information for the Internet community. This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] This document specifies a protocol useful in determining the current status of a digital certificate without requiring Certificate Revocation Lists (CRLs). Additional mechanisms addressing PKIX operational requirements are specified in separate documents. This document obsoletes RFCs 2560 and 6277. It also updates RFC 5912. This specification defines a framework for authentication and authorization in Internet of Things (IoT) environments called ACE-OAuth. The framework is based on a set of building blocks including OAuth 2.0 and the Constrained Application Protocol (CoAP), thus transforming a well-known and widely used authorization solution into a form suitable for IoT devices. Existing specifications are used where possible, but extensions are added and profiles are defined to better serve the IoT use cases. Ericsson Ericsson RISE RISE This document specifies a profile for the Authentication and Authorization for Constrained Environments (ACE) framework. It utilizes Ephemeral Diffie-Hellman Over COSE (EDHOC) for achieving mutual authentication between an ACE-OAuth client and resource server, and it binds an authentication credential of the client to an ACE-OAuth access token. EDHOC also establishes an Object Security for Constrained RESTful Environments (OSCORE) Security Context, which is used to secure communications between the client and resource server when accessing protected resources according to the authorization information indicated in the access token. This profile can be used to delegate management of authorization information from a resource-constrained server to a trusted host with less severe limitations regarding processing power and memory. RISE AB RISE AB Communications with the Constrained Application Protocol (CoAP) can be protected end-to-end at the application-layer by using the security protocol Object Security for Constrained RESTful Environments (OSCORE). Under some circumstances, two CoAP endpoints need to update their OSCORE keying material before communications can securely continue, e.g., due to approaching key usage limits. This document defines Key Update for OSCORE (KUDOS), a lightweight key update procedure that two CoAP endpoints can use to update their OSCORE keying material by establishing a new OSCORE Security Context. Accordingly, this document updates the use of the OSCORE flag bits in the CoAP OSCORE Option as well as the protection of CoAP response messages with OSCORE. Also, it deprecates the key update procedure specified in Appendix B.2 of RFC 8613. Therefore, this document updates RFC 8613. RISE AB RISE AB Object Security for Constrained RESTful Environments (OSCORE) uses AEAD algorithms to ensure confidentiality and integrity of exchanged messages. Due to known issues allowing forgery attacks against AEAD algorithms, limits should be followed regarding the number of times a specific key is used for encryption or decryption. Among other reasons, approaching key usage limits requires updating the OSCORE keying material before communications can securely continue. This document defines how two OSCORE peers can follow these key usage limits and what steps they should take to preserve the security of their communications. Ericsson AB Ericsson AB University of Glasgow RISE AB IN Groupe NIO This document specifies a CBOR encoding of X.509 certificates. The resulting certificates are called C509 certificates. The CBOR encoding supports a large subset of RFC 5280 and common certificate profiles, and it is extensible. Two types of C509 certificates are defined. One type is an invertible CBOR re-encoding of DER-encoded X.509 certificates with the signature field copied from the DER encoding. The other type is identical except that the signature is computed over the CBOR encoding instead of the DER encoding, thereby avoiding the use of ASN.1. Both types of certificates have the same semantics as X.509 while providing comparable size reduction. This document also specifies CBOR-encoded data structures for certification requests and certification request templates, new COSE headers, as well as a TLS certificate type and a file format for C509. This document updates RFC 6698 by extending the TLSA selectors registry to include C509 certificates. Ericsson AB Ericsson AB INRIA INRIA Sandelman Software Works Ephemeral Diffie-Hellman Over COSE (EDHOC) is a lightweight authenticated key exchange protocol intended for use in constrained scenarios. This document specifies Lightweight Authorization using EDHOC (ELA). The procedure allows authorizing enrollment of new devices using the extension point defined in EDHOC. ELA is applicable to zero-touch onboarding of new devices to a constrained network leveraging trust anchors installed at manufacture time. RISE AB Ericsson AB This document updates the Authentication and Authorization for Constrained Environments framework (ACE, RFC 9200) as follows. (1) It defines the Short Distribution Chain (SDC) workflow that the authorization server (AS) can use for uploading an access token to a resource server on behalf of the client. (2) For the OAuth 2.0 token endpoint, it defines new parameters and encodings and it extends the semantics of the "ace_profile" parameter. (3) For the OAuth 2.0 authz-info endpoint, it defines a new parameter and its encoding. (4) It defines how the client and the AS can coordinate on the exchange of the client's and resource server's public authentication credentials, when those can be transported by value or identified by reference; this extends the semantics of the "rs_cnf" parameter for the OAuth 2.0 token endpoint, thus updating RFC 9201. (5) It extends the error handling at the AS, for which it defines a new error code. (6) It deprecates the original payload format of error responses conveying an error code, when CBOR is used to encode message payloads. For those responses, it defines a new payload format aligned with RFC 9290, thus updating in this respect also the profiles defined in RFC 9202, RFC 9203, and RFC 9431. (7) It amends two of the requirements on profiles of the framework. Inria Ericsson Ericsson University of Murcia University of Murcia This document specifies a Pre-Shared Key (PSK) authentication method for the Ephemeral Diffie-Hellman Over COSE (EDHOC) Lightweight Authenticated Key Exchange (LAKE) protocol. The PSK method provides mutual authentication, ephemeral key exchange, identity protection, and quantum resistance while incurring lower computational costs than the public-key authentication methods specified for EDHOC. It is suited for systems where nodes share a PSK provided out-of-band (external PSK) and enables efficient session resumption with less computational overhead when the PSK is provided from a previous EDHOC session (resumption PSK). This document details the PSK message flow, key derivation changes, message formatting, processing, and security considerations.

Foreseen Advanced Processing of Incoming EDHOC message_1 As mentioned in , future developments in EDHOC and in related external security applications might rely on an EAD item in EDHOC message_1 that specifies the authentication credential CRED associated with the Initiator (by value or by reference), as wrapped in a cryptographically protected "envelope". In order to handle such a case, the processing of an incoming EDHOC message_1 as described in is extended with additional steps performed by the SPO. Such an extended side processing shares similarities with that of an incoming EDHOC message_2 or message_3 (see ). In particular, similarly to what is compiled in and , the SPO first checks whether expected EAD items are absent in message_X (see ), and then performs the following steps.

• (0) The SPO checks the presence of an EAD item that specifies the authentication credential CRED associated with the Initiator (by value or by reference). If no such EAD item is found, the SPO moves to Step 12. Otherwise, the SPO moves to Step 1.
• (1) The SPO determines CRED based on an EAD item retrieved at Step 0. The EAD item can specify CRED by value or by reference, including a URI or other external reference where CRED can be retrieved from. If CRED is already stored, the SPO moves to Step 2. Otherwise, the SPO moves to Step 3.
• (2) The SPO determines if the stored CRED is currently trusted and valid, e.g., by verifying that CRED has not expired and has not been revoked. Performing such a validation might require the SPO to first process an EAD item included in message_1. In the case that CRED is determined to be valid, the SPO moves to Step 9. Otherwise, the SPO moves to Step 11.
• (3) The SPO attempts to retrieve CRED via an EAD item considered at Step 1. Then, the SPO moves to Step 4.
• (4) If the retrieval of CRED has succeeded, the SPO moves to Step 5. Otherwise, the SPO moves to Step 11.
• (5) If the enforced trust policy for new authentication credentials is "NO-LEARNING" and P does not admit any exceptions that are acceptable to enforce for message_1 (see ), the SPO moves to Step 11. Otherwise, the SPO moves to Step 6.
• (6) If this step has been reached, the peer P is not already storing the retrieved CRED and, at the same time, it enforces either the trust policy "LEARNING" or the trust policy "NO-LEARNING" while also enforcing an exception acceptable for message_1 (see ). Consistent with that, the SPO determines if CRED is currently valid, e.g., by verifying that CRED has not expired and has not been revoked. Validating CRED might require the SPO to first process an EAD item included in message_1. After successfully validating CRED, the peer P can typically consider CRED as ultimately trusted as well. However, there can be cases where P requires to obtain additional information before doing so. If such additional information can be retrieved from message_1 (e.g., from an EAD item included therein), then P uses it to assess if CRED is trusted. Otherwise, if such additional information is expected later on during the EDHOC session, it can be acceptable for P to consider CRED as provisionally trusted. After completing the validation and trust assessment of CRED, the SPO moves to Step 7.
• (7) If CRED has been determined valid and (provisionally) trusted, the SPO moves to Step 8. Otherwise, the SPO moves to Step 11.
• (8) The SPO stores CRED as a valid and (provisionally) trusted authentication credential associated with the other peer, together with corresponding authentication credential identifiers (see ). Then, the SPO moves to Step 9.
• (9) The SPO checks if CRED is fine to use in the context of the ongoing EDHOC session, also depending on the specific identity of the other peer (see Sections and of ). If this is the case, the SPO moves to Step 10. Otherwise, the SPO moves to Step 11.
• (10) P uses CRED as authentication credential associated with the other peer in the ongoing EDHOC session. Then, the SPO moves to Step 12.
• (11) The SPO has not found a valid and (provisionally) trusted authentication credential associated with the other peer that can be used in the ongoing EDHOC session. Therefore, the EDHOC session with the other peer is aborted.
• (12) The SPO processes any EAD item included in message_1 that has not already been processed. Once all such EAD items have been processed, the SPO transfers control back to EDHOC. When doing so, the SPO also provides EDHOC with any produced EAD items to include in the EAD field of the next outgoing EDHOC message.

The flowchart in shows the different steps taken for the advanced processing of an incoming EDHOC message_1 defined above.

Processing Steps for Incoming EDHOC message_1. | CRED via |----->| retrieval | | | | | an already | | an EAD item | | of CRED | | | | | stored CRED? | +-------------+ | successful? | | | | +----------------+ +-------------+ | | | | | | | | | | | NO | YES | | | | +----------------+ | | | | | | | | | | | YES | | | | | v v v | | | +-----------------+ NO +-----------+ YES +-----------------+ | | | | 2. Is this CRED |---->| 11. Abort |<------| 5. Is the trust | | | | | still valid and | | the EDHOC | | policy used | | | | | trusted? | | session | | "NO-LEARNING", | | | | +-----------------+ | | | without any | | | | | | | | acceptable | | | | | | | | exceptions? | | | | | | | +-----------------+ | | | | YES | | | | | | v | | Here the trust | NO | | | +-----------------+ NO | | policy used is | | | | | 9. Is this CRED |---->| | "LEARNING", or | | | | | good to use in | +-----------+ "NO-LEARNING" | | | | | the context of | ^ together with | | | | | this EDHOC |<----+ | an overriding | | | | | session? | | | exception | | | | +-----------------+ | | | | | | | | | v | | | | | | +---------------+ | | | | | | | 6. Assess if | | | | | | | | CRED is valid | | | | | | | | and trusted | | | | | | | +---------------+ | | | | | | | | | | | YES | | NO | | | | | | | v | | | | | +-------------------------------+ | | | | | | 7. Is CRED valid and | | | | | | | (provisionally) trusted? | | | | | | +-------------------------------+ | | | | | | | | | | | | YES | | | v | v | | | +------------------+ | +-------------------------------+ | | | | 10. Continue by | | | 8. Store CRED as valid and | | | | | considering this | +-----| (provisionally) trusted. | | | | | CRED as the | | | | | | | authentication | | Pair CRED with consistent | | | | | credential | | credential identifiers, for | | | | | associated with | | each supported type of | | | | | the other peer | | credential identifier. | | | | +------------------+ +-------------------------------+ | | | | | | | | | | v v | | +-------------------------------------------------------------+ | | | 12. Process the EAD items that have not been processed yet. | | | | | | | | Make all the results of the EAD processing available to | | | | build the next EDHOC message. | | | +-------------------------------------------------------------+ | | | | +---------|-----------------------------------------------------------+ | | v Control transferred back to the core EDHOC processing | | v +----------------+ | Advance the | (Core EDHOC processing) | protocol state | +----------------+ ]]>

Document Updates

Version -06 to -07

• Discussed retention of completed EDHOC sessions.
• Discussed handling of incoming EDHOC error messages in a completed EDHOC session.
• Clarifications:
  • An EAD field can include one or more EAD items.
  • Use of a new EAD item in the EDHOC and OSCORE profile of ACE.
  • Table summarizing expected behavior for different trust policies.
  • Scope limited to authentication methods defined in RFC 9528.
• Consistency alignments with draft-ietf-ace-edhoc-oscore-profile.
• Editorial fixes and improvements.

Version -05 to -06

• Generalized trust assessment of authentication credentials.
• Revised discussion on the ELA procedure, based on upcoming updates expected in version -07 of draft-ietf-lake-authz.
• Added side-processing check about the absence of expected EAD items in incoming EDHOC messages.
• Added "Operational Considerations" section.
• Editorial fixes and improvements.

Version -04 to -05

• Minor clarifications.
• Editorial fixes and improvements.

Version -03 to -04

• Clarified and re-positioned exceptions to NO-LEARNING policy.
• Added security considerations.
• Appendix on foreseen advanced processing of incoming EDHOC message_1.
• Clarifications and editorial improvements.

Version -02 to -03

• Consistent use of "trust policy" instead of "trust model".
• More modular presentation of trust policies and their enforcement.
• Alignment with use of EDHOC in the EDHOC and OSCORE profile of ACE.
• Note on follow-up actions for the application after EDHOC completion.
• Removed moot section on special handling when using the EDHOC and OSCORE profile of ACE.
• Consistency checks of authentication credentials from ID_CRED and EAD items.
• Updated reference.
• Clarifications and editorial improvements.

Version -01 to -02

• Improved content on the EDHOC and OSCORE profile of ACE.
• Admit situation-specific exceptions to the "NO-LEARNING" policy.
• Using the EDHOC and OSCORE profile of ACE with the "NO-LEARNING" policy.
• Revised guidelines on using EDHOC with CoAP and Block-wise.
• Editorial improvements.

Version -00 to -01

• Added considerations on trust policies when using the EDHOC and OSCORE profile of the ACE framework.
• Placeholder section on special processing when using the EDHOC and OSCORE profile of the ACE framework.
• Added considerations on using EDHOC with CoAP and Block-wise.
• Editorial improvements.

Acknowledgments The author sincerely thanks , , , , , , , , and for their comments and feedback. The work on this document has been partly supported by the Sweden's Innovation Agency VINNOVA and the Celtic-Next project CYPRESS.